The ability to locate the position of a mobile communication unit operating in a wireless communication system provides many well known advantages. Exemplary uses of such position locating capability include security applications, emergency response applications, and travel guidance applications. Among several possible techniques for providing position locating capability, techniques that make measurements on uplink signals from the mobile unit are attractive, because they normally do not require any changes to the mobile units.
One example of an uplink time of arrival approach will now be described with respect to the Global System for Mobile Communication (GSM), which is exemplary of a wireless communication system in which uplink time of arrival techniques are applicable. When an external application (or the GSM network itself) decides to locate the position of a mobile unit (also referred to as mobile station MS), a Mobile Location Center forces (via a base station controller) the mobile unit to perform a conventional asynchronous handover, whereupon the mobile unit transmits up to 70 uplink access bursts, one burst per TDMA frame (i.e., one burst every 8 time slots). The mobile unit transmits the access bursts in an attempt to comply with the asynchronous handover command.
The Mobile Location Center (MLC) orders a number of Location Measurement Units (LMUs) to capture the access bursts and measure the time of arrival of each burst at each LMU. The LMUs then provide the MLC with their time of arrival measurements and reliability estimates for these measurements. In order to compute the position of the mobile station, the MLC uses the time of arrival values and corresponding reliability parameters, the geographic location coordinates of the LMUs, and information regarding time differences among the respective internal time bases of the LMUs. For example, each LMU can be provided with an absolute time reference (e.g., a Global Positioning System (GPS) clock), in which case the LMUs are all synchronized together, so that relative time differences among the LMUs are not a factor in the MLC's calculation of the position of the mobile station.
However, if the LMUs do not include an absolute time reference, then the relative differences among their respective local time references can be determined, for example, by having each LMU measure the time of arrival of an uplink burst from a stationary reference mobile station positioned at a known location in the network. The time of arrival information for the reference mobile station is then transmitted from the LMUs to the MLC. The MLC can use these time of arrival measurements to compute the relative differences in the timing references of the respective LMUs, also referred to as inter-LMU time differences (ILTDs).
Other conventional techniques are also available for determining the ILTDs.
Because the MLC knows the ILTDs (or alternatively knows that the LMUs are all synchronized by a GPS system), it can calculate an estimate of the position of a given mobile station from the time of arrival information provided by the LMUs, using conventional Time Difference of Arrival (TDOA) techniques.
The LMUs which should participate in a positioning measurement must be configured before they can make TOA measurements. That is, the LMUs need information about measurement starting time, measurement frequency, hopping sequence and other parameters. The MLC conventionally provides the LMUs with all the information they need to make the TOA measurements. Therefore, the MLC must select the LMUs for a certain positioning measurement prior to the measurement. To position a certain mobile station, TOA measurements must be performed by at least 3 LMUs. However, to avoid situations with poor measurement geometry and to combat low SNR (signal-to-noise ratio), it may be preferable to use more (e.g., 5-7) LMUs for the TOA measurements. On the other hand, due to capacity constraints, the MLC should preferably select as few as possible LMUs for the TOA measurements. The accuracy of a position fix depends on the accuracy of the individual TOA measurements and on the measurement geometry.
Therefore, it is important to select the LMUs such that (1) a good measurement geometry is obtained and (2) the link from the MS to the selected LMU has a good quality, e.g., a good SNR. The invention enables a mobile location node such as an MLC to select LMUs according to how well the LMUs fulfill these requirements.